This invention relates to ploughs, particularly those for burial of cables and pipelines in the bed of the sea or other water. Ploughs are well-known for use in protecting cable and pipelines underwater. The cables or pipelines are buried to protect them from damage by, for example, fishing equipment or anchors. Typically these ploughs are towed via a towrope by a vessel on the surface of the water, but may be towed by an underwater tractor. Such ploughs are fitted with a soil-engaging share that is shaped to dig into the seabed under the action of the towing force and produce a trench into which the cable or pipe is placed. The ploughs usually lay the cable or pipeline in the trench. The invention applies to all types of plough.
Hitherto, such ploughs have often been fitted with a steering system that operates by exerting transverse forces on the tow wire and thus generating couples on the plough that alter the direction of travel of the plough. Some of these ploughs are fitted with a bridle and crank mechanism to minimise the forces required to steer the plough. Such ploughs are described in patent EP 0185422. In some applications this mechanism can be lifted into an upright position to move the two wire attachment points to a position above the plough so that the same wire can lift the plough in a level attitude.
This method of steering a plough has the disadvantage that the direction of travel of the plough is affected by changes in the tow rope direction together with any side forces, for example from side slopes that the plough may be traversing. The plough or a part of it is effectively dragged sideways across the seabed.
Ploughs that steer by means of steerable fins attached to skids that support the front of the plough are also known. Such a plough is shown in EP 0010915. This design overcomes the disadvantage of the first design in that the plough can be steered relative to the seabed without being dragged across the seabed. The mechanism works in the same way as the steering in a car. To minimise the steering forces required from the steerable fins on the skids, the tow rope is connected near to the back of the plough, or to a towing mechanism with a single tow point slidably mounted on a curved arm extending lateraly from the main plough beam, to give the effect of towing from near the back—see EP 0010915.
This method of steering has the disadvantage that without such a force minimisation towing mechanism the steering angles relative to the tow rope are limited. Also the force minimisation towing mechanism described would be difficult to adapt for larger angles of steering because of the requirement for a mechanism of greater size.
A further feature of these ploughs is the need for the towing mechanism to pass over the top of the plough and therefore also, where applicable, the cable or pipeline passage through the plough. This prevents or limits one's ability to mount other equipment, particularly if the tow points must be transferred to a lifting position.